Glycolide production process, and glycolic acid oligomer for glycolide production

ABSTRACT

The invention relates to a glycolide production process wherein a depolymerization reaction system comprising a glycolic acid oligomer or a glycolic acid oligomer and a polar organic solvent is heated to depolymerize the glycolic acid oligomer into glycolide, the resulting glycolide or the glycolide and polar organic solvent are distilled out of the depolymerization reaction system, and the glycolide is recovered from distillates obtained by distillation. The glycolic acid oligomer or the glycolic acid oligomer and polar organic solvent are continuously or intermittently charged into the depolymerization reaction system, thereby carrying out depolymerization reactions continuously or repeatedly. During the depolymerization reactions, a compound having an alcoholic hydroxyl group is permitted to exist at a specific quantitative ratio in the depolymerization reaction system. The invention is also concerned with a glycolic acid oligomer for the production of glycolide, which is obtained by condensation of glycolic acid in the presence of the compound having an alcoholic hydroxyl group.

TECHNICAL FIELD

[0001] The present invention relates generally to a process for theproduction of glycolide that is a cyclic dimer ester of glycolic acid,and more particularly to a glycolide production process bydepolymerization by heating of glycolic acid oligomers, which ensuresthe long-term stability of a depolymerization reaction system comprisinga glycolic acid oligomer, so that the depolymerization reaction can beperformed in a stable yet efficient manner even when thedepolymerization reaction is carried out continuously or repeatedly.

[0002] The present invention is also concerned with a novel glycolicacid oligomer, which can reduce or substantially eliminate adverseinfluences ascribable to impurities contained in glycolic acid that isthe raw material for glycolic acid oligomers, so that thedepolymerization reaction can be carried out in a stable yet efficientfashion even when the depolymerization reaction is conductedcontinuously or repeatedly over an extended period of time.

BACKGROUND ART

[0003] Polyglycolic acid is a polyester formed bydehydration-polycondensation of glycolic acid (i.e., α-hydroxylaceticacid) and having the following formula:

[0004] Polyglycolic acid is a biodegradable polymer that is hydrolyzedin vivo and, in natural environments, is metabolized and decomposed bymicroorganisms into water and carbonic acid gas. For this reason, thepolyglycolic acid now attracts attention as environment-friendly polymersubstitutes for medical materials or general-purpose. However, it isstill difficult to obtain any polyglycolic acid having a high molecularweight by means of the dehydration-polycondensation of glycolic acid.

[0005] According to another polyglycolic acid production process so farknown in the art, glycolide of the following formula, which is a cyclicdimer ester of glycolic acid is first synthesized.

[0006] Then, this glycolide is subjected to ring-opening polymerizationin the presence of a catalyst such as stannous octoate.

[0007] To produce polyglycolic acid (also called “polyglycolide”) havinga high molecular weight by the ring-opening polymerization of glycolide,it is required to use high-purity polyglycolide as the startingmaterial. To use glycolide as the starting material to producepolyglycolic acid on an industrial scale, it is thus essential toeconomically feed such high-purity glycolide.

[0008] Glycolide is a cyclic ester compound having the structure whereintwo water molecules are eliminated from two glyclolic acid molecules.Only by the esterification reaction of glycolic acid, however, anyglycolide cannot be obtained because of the formation of glycolic acidoligomers. So far, various glycolide production processes have beenproposed.

[0009] U.S. Pat. No. 2,668,162 discloses a process in which a glycolicacid oligomer is crushed into powders and heated at 270 to 285° C. underan ultra-high vacuum (12 to 15 Torr (1.6 to 2.0 kPa)) while the powdersare fed to a reaction vessel in small portions (about 20 g/hour) fordepolymerization, and the resultant glycolide-containing vapor isentrapped. This process, albeit being suitable for small-scaleproduction, is found to have difficulty in large-scale production and sounsuitable for mass production. In addition, this process causes theoligomer to become heavy upon heating and so remain in the form of muchresidues in the reaction vessel, resulting in decreased glycolide yieldsand the need of cleaning off the residues. To add to this, the processmakes glycolide (having a melting point of 82 to 83° C.) and byproductslikely to separate out in recovery lines, ending up with troubles suchas line clogging.

[0010] U.S. Pat. No. 4,727,163 shows a glycolide production processwherein a polyether having good thermal stability is used as asubstrate, a small amount of glycolic acid is then block copolymerizedwith the substrate to obtain a block copolymer, and the block copolymeris finally heated for depolymerization. However, this blockcopolymerization process is intractable and incurs some considerableproduction cost. In addition, the process makes glycolide and byproductslikely to separate out in recovery lines, leading to troubles such asline clogging.

[0011] U.S. Pat. Nos. 4,835,293 and 5,023,349 teach a process wherein anα-hydroxycarboxylic acid oligomer such as a polyglycolic acid oligomeris heated into a melt, and a cyclic dimer esters such as glycolidegenerated and vaporized out of the surface of the melt is entrained inan inert gas such as nitrogen gas and stripped in a low-boiling solventsuch as acetone or ethyl acetate for recovery. With this process, it isstill difficult to cut back on production costs, because of problemssuch as a slow formation rate of the cyclic dimer ester, possibleformation of heavy materials in the melt, and the need for preheatingfor blowing a large amount of inert gas into the melt.

[0012] French Patent No. 2692263-A1 discloses a process for theproduction of a cyclic dimer ester wherein an oligomer of anα-hydroxycarboxylic acid or its ester or salt is added to a solvent witha catalyst added thereto, and then stirred in the presence of heat forcatalytic decomposition. This process is carried out under normal orapplied pressure, using a solvent suitable for entraining the cyclicdimer ester therein in a gaseous phase state. The gaseous phase is thencondensed for the recovery of the cyclic dimer ester and solvent. Thespecification refers to only an example wherein a lactic acid oligomeris used as the raw feed and dodecane (having a boiling point of about214° C.) is employed as the solvent. However, the results of follow-upexperimentation made by the inventors under the same conditions asdescribed in the example and using a glycolic acid oligomer and dodecaneshowed that heavy materials begin to form simultaneously with the startof the depolymerization reaction, the formation of glycolide stops at apoint of time when a very slight amount of glycolide is formed, and muchlabor is needed for cleaning reaction residues because they are tooviscous.

[0013] JP-A 09-328481 filed by the applicant of this applicationdiscloses a process comprising the steps of heating and depolymerizingan α-hydroxycarboxylic acid oligomer such as a glycolic acid oligomer ina polar organic solvent having a high boiling point, and distilling outthe resultant cyclic dimer ester such as glycolide together with thepolar organic solvent, and removing the cyclic dimer ester from thedistillates.

[0014] The results of the inventors' subsequent investigation haveshowed that if a polyalkylene glycol ether having satisfactory thermalstability is used as the polar organic solvent in this process, costreductions can then be achieved by recycling the solvent.

[0015] When a glycolic acid oligomer synthesized with a commerciallyavailable industrial-grade aqueous solution of glycolic acid isdepolymerized in a high-boiling polar organic solvent, however, it hasbeen found that the depolymerization reaction system becomes graduallyunstable and so the formation rate of glycolide becomes low. To achieveefficient mass-production of glycolide by the aforesaid process, thedepolymerization reaction should preferably be carried out continuouslyor repeatedly in the same reaction vessel.

[0016] That is, the depolymerization reaction system comprising aglycolic acid oligomer and a polar organic solvent is heated todepolymerize the glycolic acid oligomer into glycolide, and theresulting glycolide is distilled together with the polar organic solventout of the depolymerization reaction system. It is here noted that ifthe solution remaining in the depolymerization reaction system iscontinuously or intermittently replenished with a fresh glycolic acidoligomer and a fresh polar organic solvent, it is then possible to carryout the depolymerization reaction continuously or repeatedly over anextended period.

[0017] As the depolymerization reaction takes place continuously orrepeatedly within the same reaction vessel, however, the formation rateof glycolide decreases gradually, and the depolymerization reactionsystem becomes viscous with a buildup of heavier materials, resulting ina possibility that the system may boil suddenly. To be more specific,when the depolymerization reaction is repeated at least ten times withinthe same reaction vessel, it is found that there is a noticeable drop ofthe distillation rate of glycolide.

[0018] The inventors have made further studies to elucidate the cause ofthis problem. As a result of hydrolysis under alkaline conditions of aglycolic acid oligomer synthesized using a commercially availableindustrial-grade aqueous solution of glycolic acid, it has thus beenfound that such a glycolic acid oligomer contains, in addition toglycolic acid, organic acids such as diglycolic acid and methoxy aceticacid. Although depending on how to produce glycolic acid, oxalic acidmay also be detected.

[0019] When the depolymerization reaction is carried out continuously orrepeatedly while this system Is replenished with a fresh glycolic acidoligomer and a fresh polar organic solvent, these organic acids arebuilt up in the depolymerization reaction system because of theirrelatively high boiling points. The accumulation of the organic acidswithin the depolymerization reaction system has now been found to haveadverse influences on the depolymerization reaction.

[0020] The instability of the depolymerization reaction system and thedecreased formation rate of glycolide due to such organic acidimpurities may possibly be prevented by using an aqueous solution ofhigh-purity glycolic acid or purifying an industrial-grade aqueoussolution of glycolic acid thereby reducing the organic acid impuritycontent thereof. However, the glycolic acid purification step costsmuch, and so goes against glycolide production cost reductions andeventually provides an obstacle to versatile applications ofpolyglycolic acid.

[0021] In addition, even with a glycolic acid oligomer synthesized usingan aqueous solution of high-purity glycolic acid or a purified aqueoussolution of glycolic acid, the accumulation of slight amounts of organicacid impurities causes the depolymerization reaction to become graduallyinstable and the formation rate of glycolide to decrease, when thedepolymerization reaction is carried out continuously or repetitivelyover an extended period. Thus, further improvements are still needed soas to achieve stable, efficient, low-cost production of high-purityglycolide on an industrial scale.

DISCLOSURE OF THE INVENTION

[0022] One object of the present invention is to provide a process forproducing glycolide by depolymerization by heating of glycolic acidoligomers wherein, by imparting long-term stability to adepolymerization reaction system comprising a glycolic acid oligomer,depolymerization reactions can be run stably yet with efficiency, evenwhen the depolymerization reactions are carried out over an extendedperiod.

[0023] Another object of the present invention is to provide anunheard-of glycolic acid oligomer capable of reducing or substantiallyeliminating adverse influences ascribable to impurities contained inglycolic acid that is the raw material for glycolic acid oligomers, sothat de-polymerization reactions can be carried out stably yet withefficiency, even when they are run continuously or repeatedly over anextended period.

[0024] As a consequent of intensive studies made to accomplish theaforesaid objects, the inventors have now found that if, in a glycolideproduction process comprising a step of the depolymerization by heatingof a glycolic acid oligomer, a compound (A) having an alcoholic hydroxylgroup is allowed to exist in a depolymerization reaction system during adepolymerization reaction, provided that the amount of the compound (A)in the depolymerization reaction system is controlled such that thealcoholic hydroxyl group amount of the compound (A) is kept at 0.5equivalent or greater with respect to the total carboxyl group amount ofan organic acid (B) comprising diglycolic acid, methoxy acetic acid andoxalic acid formed upon hydrolysis of the depolymerization reactionsystem under alkaline conditions, it is then possible to stabilize thede polymerization reaction system over an extended period.

[0025] According to the process wherein the compound having an alcoholichydroxyl group is permitted to exist, glycolide can be obtained stablyyet with efficiency, even when the depolymerization reaction is carriedout continuously or repeatedly within the same reaction vessel.According to the production process of the invention, it is thuspossible to achieve economical mass production of glycolide on anindustrial scale.

[0026] It has also been found that by using as the raw material aglycolic acid oligomer that is obtained by the condensation of glycolicacid in the presence of the compound (A) having an alcoholic hydroxylgroup and a boiling point of 190° C. or higher as well, it is possibleto obtain glycolide stably yet with efficiency, even when thedepolymerization reaction is carried out continuously or repeatedlywithin the same reaction vessel. These findings have underlain thepresent invention.

[0027] Thus, the present invention provides a glycolide productionprocess including a step of depolymerization by heating of a glycolicacid oligomer, characterized in that:

[0028] a depolymerization reaction is carried out through the followingsteps (i) to (iv):

[0029] step (i) of heating a depolymerization reaction system comprisinga glycolic acid oligomer or a glycolic acid oligomer plus a polarorganic solvent to depolymerize the glycolic acid oligomer intoglycolide,

[0030] step (ii) of distilling the glycolide formed by depolymerizationor the glycolide and polar organic solvent out of the depolymerizationreaction system,

[0031] step (iii) of recovering the glycolide from distillates obtainedby distillation, and

[0032] step (iv) of charging the glycolic acid oligomer or the glycolicacid oligomer and polar organic solvent continuously or intermittentlyinto the depolymerization reaction system, in which:

[0033] (v) during the depolymerization reaction, a compound (A) havingan alcoholic hydroxyl group is permitted to exist in thedepolymerization reaction system, provided that the amount of thecompound (A) in the depolymerization reaction system is controlled suchthat the alcoholic hydroxyl group amount of said compound (A) is kept at0.5 equivalent or greater with respect to the total carboxyl groupamount of an organic acid (B) comprising diglycolic acid, methoxy aceticacid and oxalic acid formed upon hydrolysis of the depolymerizationreaction system under alkaline conditions.

[0034] The present invention also provides a glycolic acid oligomer forthe production of glycolide, which is obtained by the condensation ofglycolic acid in the presence of the compound (A) having an alcoholichydroxyl group and a boiling point of 190° C. or higher as well.

BEST MODE FOR CARRYING OUT THE INVENTION

[0035] 1. Depolymerization Reaction

[0036] By way of example but not by way of limitation, thedepolymerization method usable herein includes melt depolymerization asset forth typically in U.S. Pat. No. 2,668,162, solutiondepolymerization as set forth typically in U.S. Pat. No. 5,830,991 andJP-A 09-328481, and solid-phase depolymerization as set forth typicallyin JP-A 11-116666.

[0037] Thus, the “depolymerization reaction system” used herein isgenerally broken down into a system composed substantially of a glycolicacid oligomer alone, and a system comprising a glycolic acid oligomerand a polar organic solvent, depending on the depolymerization methodused.

[0038] Heating of the depolymerization reaction system composedsubstantially of a glycolic acid oligomer alone causes the glycolideresulting from the depolymerization reaction to sublimate or evaporate.It is here noted that the discharge of the sublimated or evaporatedglycolide out of the depolymerization reaction system is also referredto as “distillation”. Heating is carried out under normal pressure orreduced pressure. It is also possible to blow an inert gas into thedepolymerization reaction system, thereby carrying the resultingglycolide therewith out of the system.

[0039] As the depolymerization reaction system comprising a mixture of aglycolic acid oligomer and a polar organic solvent is heated, theglycolide resulting form the depolymerization reaction and the polarorganic solvent are co-distilled out. By separating the glycolide fromdistillates, the glycolide may be recovered. In this case, too, thedepolymerization reaction is carried out by heating the depolymerizationreaction system under normal pressure or reduced pressure.

[0040] Preferably in the present invention, the solutiondepolymerization process for depolymerizing the glycolic acid oligomerin a solution-phase state should be used in view of prevention of theglycolic acid oligomer used as the raw material from turning intoheavier material and glycolide production efficiency.

[0041] A preferable solution depolymerization process used hereinincludes the following steps (i), (ii) and (iii). At step (i) thedepolymerization reaction system comprising a glycolic acid oligomer anda polar organic solvent is heated for the depolymerization of theglycolic acid oligomer into glycolide, at step (ii) the glycolideresulting from the depolymerization and the polar organic solvent areco-distilled out of the depolymerization reaction system, and at step(iii) the glycolide is separated and recovered from the distillatesobtained by distillation.

[0042] Preferably at the aforesaid step (i), a depolymerization reactionsystem composed of a mixture comprising a glycolic acid oligomer and anorganic solvent having a boiling point of 230 to 450° C. is heated undernormal pressure or reduced pressure until the proportion of a glycolicacid oligomer melt phase remaining in the mixture is reduced down to 0.5or less. In this state, heating is continued for the depolymerization ofthe glycolic acid oligomer into glycolide. This is because thedepolymerization reaction can be carried out with efficiency. The polarorganic solvent should preferably have a molecular weight in the rangeof 150 to 450. A particularly preferred polar organic solvent is apolyalkylene glycol diether.

[0043] When a mixture comprising a glycolic acid oligomer and a polarorganic solvent is used for the depolymerization reaction system, themixture is heated to a temperature of usually 230° C. or higher undernormal pressure or reduced pressure so that the whole, or a substantial,portion of the glycolic acid oligomer is dissolved in the polar organicsolvent. More exactly, the glycolic acid oligomer is dissolved in thepolar organic solvent until the proportion of the glycolic acid oligomermelt phase in the mixture is reduced down to 0.5 or lower.

[0044] Where a substantial portion of the glycolic acid oligomer is notdissolved in the polar organic solvent, the proportion of the oligomermelt phase becomes too high to distill out glycolide. In addition, heavymaterial-formation reactions are likely to occur in the oligomer meltphase. By depolymerizing the glycolic acid oligomer in a solution state,the formation rate of glycolide generated and vaporized out of thesurface of the oligomer can be much more increased. In this regard, itis preferable that upon the mixture heated to a temperature at whichdepolymerization occurs, the glycolic acid oligomer is alreadycompletely dissolved in the polar organic solvent; the melt phase iskept against any phase separation.

[0045] It is here noted that the term “proportion of the glycolic acidoligomer melt phase” remaining in the mixture refers to the volume ratioof the oligomer melt phase formed in an actually used polar organicsolvent with the proviso that the volume of the oligomer melt phaseformed in a solvent such as liquid paraffin, in which the glycolic acidoligomer is substantially insoluble, is 1.

[0046] When the sole use of the polar organic solvent is found to makethe solubility of the glycolic acid oligomer insufficient, it may beenhanced by conducting heating in the presence of the compound (A)having an alcoholic hydroxyl group.

[0047] Heating is carried out under normal pressure or reduced pressure;however, it should preferably be done under a reduced pressure of about0.1 to 90 kPa. Heating should desirously be performed in an inertatmosphere. The mixture is heated to at least 230° C., at which thedepolymerization reaction of glycolic acid oligomer occurs. However,usually, the mixture is heated to temperatures in the range of 230 to320° C., preferably 235 to 300° C., and more preferably 240 to 290° C.

[0048] By heating, the depolymerization of glycolic acid oligomer takesplace, and the resulting glycolide is co-distilled out together with thepolar organic solvent. Since the resultant glycolide is co-distilled outtogether with the organic solvent, it is possible to prevent anydeposition of glycolide on the wall surfaces of a reaction vessel andlines, which may otherwise cause accumulation of glycolide. Thedistillates are then guided out of the depolymerization reaction systemto recover glycolide therefrom. The distillates are cooled, if required,with a nonsolvent added thereto, so that glycolide is separated out andsolidified. The separated-out glycolide is isolated from the motherliquor by means of filtration, centrifugal sedimentation, decantation,etc. If required, the isolated glycolide is washed or extracted with anonsolvent such as cyclohexane or ether, and then recrystallized withethyl acetate or the like. The glycolide may also be purified bydistillation.

[0049] The mother liquor, from which glycolide has been isolated, may berecycled without any purification. Alternatively, the mother liquor maybe filtered out and purified by treatment with activated carbon, etc.for recycling purposes. Still alternatively, the mother liquor may bepurified by redistillation for recycling purposes.

[0050] As glycolide is co-distilled out of the depolymerization reactionsystem together with the polar organic solvent, there is a decrease inthe volume of the depolymerization reaction system. If fresh amounts ofthe glycolic acid oligomer and polar organic solvent—that make up forthe amount of distillates—are additionally fed to the reaction system,it is then possible to carry out the depolymerization reaction in acontinuous or repeated fashion over an extended period of time.According to the production process of the invention, thedepolymerization reaction can be carried out in a stable manner, and sosuch a process can be used, thereby making striking improvements inproduction efficiency and cutting back on cost.

[0051] More specifically according to the invention, a mixturecontaining a glycolic acid oligomer and a polar organic solvent ischarged into a reaction vessel to start the depolymerization reaction.With the progress of the depolymerization reaction, the resultingglycolide and polar organic solvent are co-distilled out of thedepolymerization reaction system. If fresh amounts of the glycolic acidoligomer and polar organic solvent are added to the reaction solutionremaining in the reaction vessel, the depolymerization reaction can thenbe carried out in a repetitive manner. Alternatively, thedepolymerization reaction may be continuously carried out while thefresh amounts of the glycolic acid oligomer and polar organic solventare supplied during the depolymerization reaction. The glycolic acidoligomer and polar organic solvent may be supplied into thedepolymerization reaction system in a continuous or intermittentfashion, or in a combined continuous and intermittent fashion. It ishere noted that the reaction solution remaining in the reaction vesselmay be wholly or partly used.

[0052] In the present invention, the depolymerization reaction iscarried out in a continuous or repetitive manner while the glycolic acidoligomer or the glycolic acid oligomer plus polar organic solvent iscontinuously or intermittently supplied into the depolymerizationreaction system, and the depolymerization is effected in the state wherethe predetermined amount of the compound having an alcoholic hydroxylgroup is always present in the depolymerization reaction system.

[0053] In order to carry out the depolymerization reaction within thesame reaction vessel in a continuous or repetitive fashion, it isnecessary that fresh amounts of the glycolic acid oligomer or theglycolic acid oligomer plus polar organic solvent be suppliedcontinuously or intermittently into the depolymerization reactionsystem. Consequently, organic acid impurities are gradually built upwithin the depolymerization reaction system. In the present invention,the compound having an alcoholic hydroxyl group is allowed to beconstantly present in the depolymerization reaction system in an amountcorresponding to the amount of the thus built up organic acidimpurities.

[0054] The amount of the compound (A) having an alcoholic hydroxylgroup, which is permitted to exist in the depolymerization reactionsystem, is such that the alcoholic hydroxyl group amount is at least 0.5equivalent, preferably at least 0.9 equivalent, and more preferably atleast 1 equivalent with respect to the total carboxyl group amount ofthe organic acid (B) comprising diglycolic acid, methoxy acetic acid andoxalic acid formed upon hydrolysis of the depolymerization reactionsystem under alkaline conditions.

[0055] When the amount of the compound (A) having an alcoholic hydroxylgroup, which is present in the depolymerization reaction system, is toosmall or when the alcoholic hydroxyl group amount of the compound (A)relative to the total carboxyl group amount of the organic acid (B) isless than 0.5 equivalent, it is difficult to control decreases in theformation rate of glycolide. When the amount of the compound (A) havingan alcoholic hydroxyl group is too large, it is often likely that thedepolymerization reaction system may be diluted, resulting indepolymerization reaction efficiency drops. In most cases, thedepolymerization reaction system may be stabilized by keeping thealcoholic hydroxyl group amount of the compound (A) relative to thetotal carboxyl group amount of the organic acid (B) in the range of 0.5to 1.5 equivalents. It is noted, however, that when the compound (A)having an alcoholic hydroxyl group takes another role of a solubilizerfor improving the solubility of the glycolic acid oligomer in the polarorganic solvent, it may be allowed to exist in the depolymerizationreaction system in an amount of up to 100 parts by weight, and morepreferably up to 50 parts by weight per 100 parts by weight of theglycolic acid oligomer.

[0056] By permitting the compound having an alcoholic hydroxyl group toexist within the depolymerization reaction system at a proportion ofpredetermined or larger equivalents relative to the carboxyl groupamount of organic acid impurities, the depolymerization reaction systemis so stabilized that any decrease in the formation rate of glycolidecan be reduced. Any detailed reason for this has yet to be clarified. Asthe depolymerization reaction takes place continuously or repetitively,there is an increase in the amount of organic acid impuritiesaccumulated in the depolymerization reaction system. When the ratio(b/a) between the terminal carboxyl group amount (b) of the organic acidimpurities present in the depolymerization reaction system and thealcoholic hydroxyl group amount (a) is shifted to an increasing value, acondensation reaction competitive with the depolymerization reactiontends to occur. This is believed to lead to a decrease in the formationrate of glycolide. The compound (A) having an alcoholic hydroxyl groupwould react or otherwise combine chemically with the organic acidimpurities, thereby reducing such a decrease.

[0057] How to add the compound (A) having an alcoholic hydroxyl groupinto the depolymerization reaction system is not particularly critical.For instance, the necessary amount of the compound (A) having analcoholic hydroxyl group may be supplied to a reaction vesselsimultaneously with, or before or after, the feed of the startingglycolic acid oligomer thereto. Alternatively, if the depolymerizationreaction is carried out with the continuous or intermittent addition ofthe starting glycolic acid oligomer while an excess amount of thecompound (A) having an alcoholic hydroxyl group has been added into thereaction vessel, it is then possible to add the compound (A) having analcoholic hydroxyl group continuously or intermittently into thedepolymerization reaction system before the total carboxyl group amountof the organic acid impurities in the depolymerization reaction systemreaches the predetermined or greater equivalent weight with respect tothe alcoholic hydroxyl group amount of the originally added compound (A)having an alcoholic hydroxyl group.

[0058] Where the depolymerization reaction is carried out repeatedlywithin the same reaction vessel, it is acceptable that when the freshamounts of the glycolic acid oligomer or the glycolic acid oligomer pluspolar organic solvent are supplied, the necessary amount of the compound(A) having an alcoholic hydroxyl group is added or a batch of thecompound (A) is added upon the number of repetition of thedepolymerization reaction reaching a constant or greater.

[0059] With the solution depolymerization process, the necessary amountof the compound having an alcoholic hydroxyl group may be added in theform of a solution wherein the glycolic acid oligomer and thepredetermined amount of the compound having an alcoholic hydroxyl groupare dissolved in the polar organic solvent. The amount of the compound(having an alcoholic hydroxyl group) used may be such that after theaddition of said compound, the alcoholic hydroxyl group amount is at apredetermined or greater equivalent ratio (0.5 equivalent or more) withrespect to the total carboxyl group amount of the organic acidimpurities in the de polymerization reaction system. Practically,however, it is preferable to previously measure the total carboxyl groupamount of the organic acid impurities contained in the starting glycolicacid oligomer. More exactly, the amount of the compound having analcoholic hydroxyl group should be controlled in such a way that therate of the alcoholic hydroxyl group amount with respect to that totalcarboxyl group amount is kept in the range of at least 0.5 equivalents,and preferably at 1.0 equivalents. At an equivalent ratio of less than0.5, the alcoholic hydroxyl group amount in the depolymerizationreaction system decreases and so the effect of the compound on thereduction of a decrease in the formation rate of glycolide does hardlymanifest itself.

[0060] Instead of adding the compound (A) having an alcoholic hydroxylgroup directly into the depolymerization reaction system, it is hereinalso possible to use as the starting glycolic acid oligomer a glycolicacid oligomer that is obtained by the condensation of glycolic acid inthe presence of the compound (A) having an alcoholic hydroxyl group anda boiling point of at least 190° C. as well. By use of such a glycolicacid oligomer, the compound having an alcoholic acid, present in thedepolymerization reaction system, may be kept in the predeterminedquantitative range. Using as the raw material a glycolic acid oligomerthat is obtained by co-condensation with the compound having analcoholic hydroxyl group is preferable, because any separate addition ofthe compound having an alcoholic hydroxyl group is not always needed andso the depolymerization reaction operation is simplified.

[0061] Such a glycolic acid oligomer may be prepared by theco-condensation of glycolic acid in the presence of the compound (A)having an alcoholic hydroxyl group and under ordinarily availablecondensation conditions. In the co-condensates, the compound having analcoholic hydroxyl group exists by itself or, alternatively, it existsin the glycolic acid oligomer, with a structure esterified byco-condensation with glycolic acid, diglycolic acid, methoxy acetic acidor oxalic acid. This would in turn reduce the actions of organic acidimpurities.

[0062] Referring here to the amount of the compound (A) having analcoholic hydroxyl group, which is allowed to coexist upon thecondensation of glycolic acid, the alcoholic hydroxyl group amountshould preferably be in the range of 0.5 to 1.5 equivalents with respectto the carboxyl group amount of the organic acid (B) comprisingdiglycolic acid, methoxy acetic acid and oxalic acid formed upon thehydrolysis under alkaline conditions of the glycolic acid oligomerobtained by condensation. When a glycolic acid oligomer obtained by thecondensation of glycolic acid while the amount of the coexistingalcoholic hydroxyl group is set at greater than 1.5 equivalents withrespect to the total carboxyl group amount of the organic acidimpurities is added as the raw material, it is acceptable to dilute thatcompound (A) with a glycolic acid oligomer not subjected tocondensation, thereby controlling the equivalent ratio of the alcoholichydroxyl group in the aforesaid range.

[0063] Referring then to the amount of the compound (A) having analcoholic hydroxyl group, which is permitted to coexist upon thecondensation of glycolic acid, the alcoholic hydroxyl group amountshould be in the range of preferably 0.5 to 1.5 equivalents, morepreferably 0.8 to 1.2 equivalents, and even more preferably 0.9 to 1.1equivalents with respect to the total carboxyl group amount of theorganic acid (B) comprising diglycolic acid, methoxy acetic acid andoxalic acid formed upon the hydrolysis under alkaline conditions of theglycolic acid oligomer produced. The total carboxyl group amount of theorganic acid (B) comprising diglycolic acid, methoxy acetic acid andoxalic acid formed upon the hydrolysis under alkaline conditions of theglycolic acid oligomer may here be considered substantially equal to thetotal carboxyl group amount of the organic acids contained in thestarting glycolic acid.

[0064] 2. Glycolic Acid Oligomer

[0065] Glycolic acid oligomers used herein may be synthesized by thecondensation of glycolic acid that may be in the form of ester (e.g.,lower alkyl esters) or salt (e.g., sodium salt).

[0066] Glycolic acid (inclusive of its ester or salt) is heated to at atemperature of usually 100 to 250° C., and preferably 140 to 230° C.under reduced or applied pressure and, if required, in the presence of acondensation catalyst or an ester exchange catalyst, so that acondensation reaction or an ester exchange reaction is carried out untildistillation of low-molecular-weight materials such as water and alcoholis not substantially found. After the completion of the condensationreaction or ester exchange reaction, the resulting glycolic acidoligomer may be used immediately as the raw material in the presentinvention. If the resulting glycolic acid oligomer discharged out of thereaction system is washed with a non-solvent such as benzene or toluene,unreacted or low-molecular-weight materials, the catalyst or the likecan then be removed therefrom. The glycolic acid oligomer used may be ina ring or straight-chain form, and the degree of polymerization is notparticularly critical. In consideration of glycolide yields upon thedepolymerization reaction, however, the glycolic acid oligomer usedshould have a melting point (Tm) of usually 140° C. or higher,preferably 160° C. or higher, and more preferably 180° C. or higher. The“Tm” used herein is understood to mean a melting point of a sample asdetected with a differential scanning calorimeter (DSC) while the sampleis heated at a heating rate of 10° C./minute in an inert gas atmosphere.

[0067] As already explained, the glycolic acid oligomer obtained by thecondensation of commercially available glycolic acids includes, inaddition to glycolic acid as a monomer unit, slight amounts of impurecomponents, for instance, diglycolic acid, methoxy acetic acid, oxalicacid and so on. Of these impurities, organic acid impurities haverelatively high boiling points. The organic acid impurities, even whencontained in slight amounts, build up in the depolymerization reactionsystem as the de-polymerization reaction proceeds continuously orrepetitively with the successive addition of the starting glycolic acidoligomer into the depolymerization reaction system, and consequentlyhave adverse influences on the depolymerization reaction.

[0068] By the coexistence of the compound (A) having an alcoholichydroxyl group according to the production process of the invention, theadverse influences of these organic acid impurities can be strikinglyreduced or substantially eliminated. For this reason, no particularlimitation is imposed on the glycolic acid oligomer used; for instance,it is possible to use inexpensive glycolic acid oligomers that areobtained by the dehydration-condensation of commercially available,industrial-grade aqueous glycolic acid solutions.

[0069] In glycolic acid oligomers obtained by conventionaldehydration-condensation processes of commercially available,industrial-grade glycolic acids, there are organic acids comprisingdiglycolic acid, methoxy acetic acid and oxalic acid formed upon theircomplete hydrolysis under alkaline conditions, the total carboxyl groupamount of which is usually at least 0.5 molt with respect to theglycolic acid. Still, the present invention makes it possible to useglycolic acid oligomers that contain a large amount of carboxyl groupsresulting from such organic acid impurities.

[0070] A glycolic acid oligomer obtained by the condensation of glycolicacid in the presence of the compound having an alcoholic hydroxyl groupand a boiling point of 190° C. or higher as well may be prepared bycondensation under the production conditions for glycolic acid oligomersand in the presence of the predetermined amount of the compound havingan alcoholic hydroxyl group.

[0071] 3. Compound Having an Alcoholic Hydroxyl Group

[0072] The compound (A) having an alcoholic hydroxyl group, used in thepresent invention, should be permitted to exist in the depolymerizationreaction system during the depolymerization reaction. Acid impurities ordiglycolic acid, methoxy acetic acid and oxalic acid have all relativelyhigh boiling points, and so are hardly removed out of thedepolymerization reaction system by distillation under de-polymerizationreaction conditions. It is thus preferable that the compound (A) havingan alcoholic hydroxyl group, too, be hardly removed out of the system bydistillation.

[0073] Specific examples of such a compound (A) having an alcoholichydroxyl group are mono-, di- or poly-hydric alcohols (inclusive oftheir partially esterified or etherified products), phenols, etc. Inparticular, the alcohols are the most effective compounds having analcoholic hydroxyl group; however, the most preference is given topolyhydric alcohols having at least two alcoholic hydroxyl groups permolecule. Even some low-molecular-weight polyhydric alcohols are moreeffective in small amounts than monohydric alcohols because of theirmerit of being hardly distilled out of the depolymerization reactionsystem. The compound (A) should have a boiling point of preferably atleast 190° C., and more preferably at least 195° C.

[0074] Where the compound (A) having an alcoholic hydroxyl group isdistilled out of the depolymerization reaction system, it is preferableto make up for losses by distillation.

[0075] Among the compounds (A) having an alcoholic hydroxyl gorup,preference is given to alkylene diols (p=1) or polyalkylene glycols(p≧2) and polyalkylene glycol monoethers having the following formulae(1) and (2), respectively, as well as tri- or poly-hydric alcohols suchas glycerin, higher alcohols such as tridecanol, etc.

[0076] Here R¹ is a methylene group or a straight-chain orbranched-chain alkylene group having 2 to 10 carbon atoms and p is aninteger of 1 or greater provided that when p is an integer of 2 orgreater, a plurality of R¹s may be identical with or different from eachother.

[0077] Here R² is a methylene group or a straight-chain orbranched-chain alkylene group having 2 to 10 carbon atoms, X¹ is ahydrocaron group and q is an integer of 1 or greater provided that whenq is an integer of 2 or greater, a plurality of R²s may be identicalwith or different from each other. Among these, preference is given toalkylene diols, polyalkylene glycols and polyalkylene glycol monoethers.

[0078] The alkylene diol (i.e., alkylene glycol) used herein, forinstance, includes ethylene glycol, propylene glycol, butylene glycol,hexanediol, hexylene glycol, hexamethylene glycol and decanediol, amongwhich ethylene glycol is preferred.

[0079] The polyalkylene glycol usable herein, for instance, includespolyethylene glycol, polypropylene glycol and polybutylene glycol.

[0080] The polyalkylene glycol monoether usable herein, for instance,includes polyethylene glycol monoalkyl ethers such as polyethyleneglycol monopropyl ether, polyethylene glycol monobutyl ether,polyethylene glycol monohexyl ether, polyethylene glycol monooctylether, polyethylene glycol monodecyl ether and polyethylene glycolmonolauryl ether, and polypropylene glycol monoalkyl ethers orpolybutylene glycol monoalkyl ethers wherein the ethyleneoxy groups inthe aforesaid compounds are substituted by propyleneoxy or butyleneoxygroups.

[0081] These compounds (A) having an alcoholic hydroxyl group may beused alone or in combination of two or more. The compounds (A) may alsobe used in combination with a glycolic acid oligomer obtained byco-condensation with a compound having an alcoholic hydroxyl group.

[0082] 4. Polar Organic Solvent

[0083] When the depolymerization reaction is carried out by the solutiondepolymerization process, the polar organic solvent is used. The polarorganic solvent is not only used as a solvent for the depolymerizationreaction but is also co-distilled out together with the resultingglycolide so that the glycolide can be discharged out of thedepolymerization reaction system. The polar organic solvent shouldpreferably have a boiling point of 230 to 450° C. and a molecular weightin the range of 150 to 450.

[0084] When a polar organic solvent having too low a boiling point isused, no high depolymerization reaction temperature can be set so thatthe rate of formation of glycolide becomes low. When a polar organicsolvent having too high a boiling point, on the other hand, the organicsolvent is hardly distilled out upon the depolymerization reaction, andso the co-distillation of the polar organic solvent and the glycolideformed by depolymerization becomes difficult. The boiling point of thepolar organic solvent used should be in the range of preferably 235 to450° C., more preferably 260 to 430° C., and most preferably 280 to 420°C.

[0085] When the molecular weight of the polar organic solvent useddeviates from the aforesaid range, the co-distillation of the polarorganic solvent and glycolide becomes difficult. The molecular weight ofthe polar organic solvent used should be in the range of preferably 180to 420, and more preferably 200 to 400.

[0086] The polar organic solvent, for instance, includes aromaticdicarboxylic acid diesters, aliphatic dicarboxylic acid diesters, andpolyalkylene glycol diethers. The aromatic dicarboxylic diester usedherein, for instance, includes phthallic esters such as dibutylphthalate, dioctyl phthalate, dibenzyl phthalate and benzylbutylphthalate and benzoic esters such as benzyl benzoate.

[0087] The aliphatic dicarboxylic diester used herein, for instance,includes adipic esters such as octyl adipate and sebacic esters such asdibutyl sebacate.

[0088] The polyalkylene glycol diether has the following formula (3):

[0089] Here R³ is a methylene group or a branched-chain orstraight-chain alkylene group having 2 to 8 carbon atoms, X² and Y areeach a hydrocarbon group, and r is an integer of 1 or greater providedthat when r is an integer of 2 or greater, a plurality of R³s may beidentical with or different from each other.

[0090] Exemplary such polyalkylene glycol diethers are polyethyleneglycol dialkyl ethers such as diethylene glycol dibutyl ether,diethylene glycol dihexyl ether, diethylene glycol dioctyl ether,triethylene glycol dimethyl ether, triethylene glycol diethylene ether,triethylene glycol dipropyl ether, triethylene glycol dibutyl ether,triethylene glycol dihexyl ether, triethylene glycol dioctyl ether,tetraethylene glycol dimethyl ether, tetraethylene glycol diethyl ether,tetraethylene glycol dipropyl ether, tetraethylene glycol dibutyl ether,tetraethylene glycol dihyexyl ether, tetraethylene glycol dioctyl ether,diethylene glycol butylhexyl ether, diethylene glycol butyloctyl ether,diethylene glycol hexyloctyl ether, triethylene glycol butylhexyl ether,triethylene glycol butyloctyl ether, triethylene glycol hexyloctylether, tetraethylene glycol butylhexyl ether, tetraethylene glycolbutyloctyl ether and tetraethylene glycol hexyloctyl ether as well aspolyalkylene glycol dialkyl ethers wherein the ethyleneoxy groups inthese compounds are substituted by propyleneoxy or butyleneoxy groups,e.g., polypropylene glycol dialkyl ethers or polybutylene glycol dialkylethers; polyethylene glycol alkylaryl ethers such as diethylene glycolbutylphenyl ether, diethylene glycol hexylphenyl ether, diethyleneglycol octylphenyl ether, triethylene glycol butylphenyl ether,triethylene glycol hexylphenyl ether, triethylene glycol octylphenylether, tetraethylene glycol butylphenyl ether, tetraethylene glycolhexylphenyl ether and tetraethylene glycol octylphenyl ether,polyethylene glycol alkylaryl ethers wherein the hydrogen groups in thephenyl groups in these compounds are substituted by an alkyl group, analkoxy group or a halogen atom, and polyalkylene glycol alkylaryl etherssuch as polypropylene glycol alkylaryl ethers or polybutylene glycolalkylaryl ethers containing propyleneoxy or butyleneoxy groups insteadof the ethyleneoxy groups in these compounds; polyethylene glycol diarylethers such as diethylene glycol diphenyl ether, triethylene glycoldiphenyl ether, tetraethylene glycol diphenyl ether, polyethylene glycoldiaryl ethers wherein the phenyl groups in these compounds aresubstituted by an alkyl group, an alkoxy group, a halogen atom, etc.,and polyalkylene glycol diaryl ethers such as polypropylene glycoldiaryl ethers or polybutylene glycol diaryl ethers containingpropyleneoxy or butyleneoxy groups instead of the ethyleneoxy groups inthese compounds.

[0091] These polar organic solvents are used in an amount of, based on amass basis, usually 0.3 to 50 times, preferably 0.5 to 20 times and morepreferably 1 to 10 times as large as the glycolic acid oligomer.

EXAMPLES

[0092] By way of example but not by way of limitation, the presentinvention is now explained more specifically with reference to synthesisexamples, inventive examples and a comparative example.

[0093] (1) Quantitative Determination Of Diglycolic Acid, MethoxyAcetate, and Oxalic Acid

[0094] A sample (5.8 grams) was placed in a 200-ml beaker with theaddition of 4 grams of NaOH and 40 grams of distilled water thereto,wherein perfect hydrolysis was carried out at 40° C. under agitation for12 to 48 hours. After regulated by sulfuric acid to pH 4.7, theresulting hydrolysis solution was provided with distilled water in sucha way as to give a total amount of 80 grams. This testing solution (2grams) was diluted with distilled water to 50 ml, 2 μl of which wereanalyzed by high performance liquid chromatography (HPLC) under thefollowing conditions.

[0095] The contents of glycolic acid, diglycolic acid, methoxy aceticacid and oxalic acid were found by the absolute calibration curve methodon the basis of calibration curves determined ahead from the respectivestandard substances.

[0096] The contents of diglycolic acid, methoxy acetic acid and oxalicacid are all given by moles in a solution state, and in terms of molt aswell.

[0097] Analytical Conditions for HPLC

[0098] Apparatus: L-6200, Hitachi, Ltd.

[0099] Column: Intersil ODS-3V (5 μm), 250×4.6 mm I.D.

[0100] Flow Rate: 1.0 mL/min.

[0101] Eluate: Aqueous solution of 0.1 M ammoniumdihydrogen-phosphate+phosphoric acid

[0102] Oven Temperature: 40° C.

[0103] Detection Condition: UV 210 nm

Synthesis Example 1 Synthesis of Glycolic Acid Oligomer (a)

[0104] A 5-liter autoclave was charged with 3,500 grams of acommercially available 70% aqueous solution of glycolic acid (of theindustrial grade, Du Pont). While stirred at normal pressure, thesolution was heated from 170° C. up to 200° C. over 2 hours for acondensation reaction during which the formed water was distilled out.Then, the internal pressure of the autoclave was lowered to 5.0 kPa, atwhich the solution was heated at 200° C. for 2 hours to distill offlow-boiling materials including unreacted matters, thereby preparing1,700 grams of glycolic acid oligomer (a). The proportions of diglycolicacid, methoxy acetic acid and oxalic acid in the glycolic acid oligomer(a) were found through an analysis by hydrolysis under alkalineconditions to be 1.0 mol %, 0.5 mol % and 0 mol %, respectively, permole of glycolic acid.

Synthesis Example 2 Synthesis of Glycolic Acid Oligomer (b)

[0105] As in synthesis example 1, 1,650 grams of glycolic acid oligomer(b) were obtained with the exception that the aqueous solution ofglycolic acid was changed to 3,570 grams of a commercially available 70%aqueous solution of high-purity glycolic acid (of no industrial grade,Du Pont) with the addition thereto of 32 grams of oxalic acid having amolecular weight of 90. The proportions of diglycolic acid, methoxyacetic acid and oxalic acid in the glycolic acid oligomer (b) were foundthrough an analysis by hydrolysis under alkaline conditions to be 0 mol%, 0 mol % and 1.25 mol %, respectively, per mole of glycolic acid.

Synthesis Example 3 Synthesis of Glycolic Acid Oligomer (c)

[0106] A 5-liter autoclave was charged with 3,500 grams of acommercially available 70% aqueous solution of glycolic acid (of theindustrial grade, Du Pont) and 250 grams of lauryl triethylene glycolhaving a molecular weight of 318.5. While stirred at normal pressure,the solution was heated from 170° C. up to 200° C. over 2 hours for acondensation reaction during which the formed water was distilled out.Then, the internal pressure of the autoclave was lowered to 5.0 kPa, atwhich the solution was heated at 200° C. for 2 hours to distill offlow-boiling materials including unreacted matters, thereby preparing1,750 grams of glycolic acid oligomer (c). The proportions of diglycolicacid, methoxy acetic acid and oxalic acid in the glycolic acid oligomer(c) were found through an analysis by hydrolysis under alkalineconditions to be 1.0 mol %, 0.5 mol % and 0 mol %, respectively, permole of glycolic acid.

[0107] This synthesis example 3 is tantamount to an example of theinvention relating to the glycolide-producing glycolic acid oligomerthat is obtained by the condensation of glycolic acid in the presence ofthe compound (A) having an alcoholic hydroxyl group and a boiling pointof 190° C. or higher as well.

Synthesis Example 4 Synthesis of Polyalkylene Glycol Ether

[0108] Commercially available polyethylene glycol dimethyl ether #250(made by Merck) was distilled to obtain tetraethylene glycol dimethylether having a polymerization degree of 4 (hereinafter abbreviated asTEGDME). The TEGDME was used as the polar organic solvent for thesolution depolymerization process.

Example 1

[0109] A 500-ml flask was charged with 100 grams of glycolic acidoligomer (a) obtained in synthesis example 1, 200 grams of TEGDME actingas the polar organic solvent and 42 grams (corresponding to 0.28 mole ofalcoholic hydroxyl group) of polyethylene glycol #300 having an averagemolecular weight of 300 and a boiling point of 410 to 470° C.(hereinafter referred to as PEG#300). The flask was heated up to 260° C.while the inside pressure was lowered to 8.0 kPa. Glycolic acid oligomer(a) was put in a dissolved state, with the proportion of its remainingmelt phase being substantially zero. A solution distilled out of thedepolymerization reaction system was cooled with ice water forentrapment.

[0110] After a five-hour reaction, the amount of the distilled-outsolution was 210 grams. From this solution, 58 grams of glycolide and150 grams of TEGDME were recovered. The distillation rate of glycolidewas 11 to 12 grams/hour. With 60 grams of glycolic acid oligomer (a) and150 grams of TEGDME added to the reaction solution remaining in theflask, the depolymerization reaction was carried out under the sameconditions as mentioned above.

[0111] In this way, the depolymerization reaction was repetitivelycarried out with the addition of glycolic acid oligomer (a) in theweight corresponding to the amount of glycolide obtained in thedistilled-out solution and TEGDME in the same amount as that of TEGDMEin the distilled-out solution.

[0112] The depolymerization reaction was repeated ten times. The feed ofglycolic acid oligomer (a) added up to 640 grams. At the time when thedepolymerization was repeated ten times, the distillation rate ofglycolide started to become low. More exactly, the distillation rate ofglycolide decreased down to 8 to 9 grams/hour. Then, the 11thdepolymerization reaction was carried out with the addition of 4 gramsof PEG#300 to 60 grams of glycolic acid oligomer (a). Consequently, thedistillation rate of glycolide again went back to the initialdistillation rate of 11 to 12 grams/hour. Likewise, the 12th to 15thdepolymerization reactions were successively carried out with theaddition of 60 grams of glycolic acid oligomer (a), 4 grams of PEG#300and 150 grams of TEGDME. Subsequently, the 16th to 20th depolymerizationreactions were successively carried out with the addition of 60 grams ofglycolic acid oligomer (a), 8.3 grams of lauryl triethylene glycolhaving a molecular weight of 318.5 and a boiling point of 450° C. orhigher and 150 grams of TEGDME, whereupon the 21st to 25thdepolymerization reactions were successively done with the addition of60 grams of glycolic acid oligomer (a), 1.3 grams of ethylene glycolhaving a molecular weight of 62 and a boiling point of 197° C. and 150grams of TEGDME. At the 11th through the 25th depolymerizationreactions, the distillation rate of glycolide was kept stable at 11 to12 grams/hour, indicating that the stable yet efficient depolymerizationreactions could be carried out over an extended period.

[0113] Upon the completion of the 25th reaction, the feed of glycolicacid oligomer (a) added up to 1,540 grams. The depolymerization reactionsystem (the reaction solution remaining in the flask) was hydrolyzedunder alkaline conditions to analyze the contents of diglycolic acid,methoxy acetic acid and oxalic acid present therein. As a result, therespective contents were found to be 0.26 mole, 0.13 mole and 0 mole.The amount of carboxyl groups in these organic acids totaled 0.65 mole.The amount of the compound having an alcoholic hydroxyl group, added atthe initial stage and the subsequent stages, was 62 grams for PEG#300,41.5 grams for lauryl triethylene glycol and 6.5 grams for ethyleneglycol, and the amount of the respective alcoholic hydroxyl groupstotaled 0.65 mole.

Example 2

[0114] A 500-ml flask was charged with 100 grams of glycolic acidoligomer (b) obtained in synthesis example 2, 200 grams of TEGDME actingas the polar organic solvent and 42 grams (corresponding to 0.28 mole ofalcoholic hydroxyl group) of PEG#300 having an average molecular weightof 300 and a boiling point of 410 to 470° C. The flask was heated up to260° C. while the inside pressure was lowered to 8.0 kPa. Glycolic acidoligomer (b) was put in a dissolved state. A solution distilled out ofthe depolymerization reaction system was cooled with ice water forentrapment.

[0115] After a five-hour reaction, the amount of the distilled-outsolution was 208 grams. From this solution, 59 grams of glycolide and149 grams of TEGDME were recovered. The distillation rate of glycolidewas 11 to 12 grams/hour. With 60 grams of glycolic acid oligomer (b) and150 grams of TEGDME added to the reaction solution remaining in theflask, the depolymerization reaction was carried out under the sameconditions as mentioned above. In this way, the depolymerizationreaction was repetitively carried out with the addition of glycolic acidoligomer (b) in the weight corresponding to the amount of glycolideobtained in the distilled-out solution and TEGDME in the same amount asthat of TEGDME in the distilled-out solution. The depolymerizationreaction was repeated ten times. The feed of glycolic acid oligomer (b)added up to 640 grams. At the 10th depolymerization reaction, thedistillation rate of glycolide decreased down to 9 to 10 grams/hour.Then, the 11th depolymerization reaction was carried out with theaddition of 4 grams of PEG#300 to 60 grams of glycolic acid oligomer(b). Consequently, the distillation rate of glycolide again went back tothe initial distillation rate of 11 to 12 grams/hour. Likewise, the 12thto 25th depolymerization reactions were successively carried out withthe addition of 60 grams of glycolic acid oligomer (b), 4 grams ofPEG#300 and 150 grams of TEGDME. At the 11th through the 25thdepolymerization reactions, the distillation rate of glycolide was keptstable at 11 to 12 grams/hour, indicating that the stable yet efficientdepolymerization reactions could be carried out over an extended period.

[0116] Upon the completion of the 25th reaction, the feed of glycolicacid oligomer added up to 1,540 grams. The depolymerization reactionsystem was hydrolyzed under alkaline conditions to analyze the contentsof diglycolic acid, methoxy acetic acid and oxalic acid present therein.As a result, the respective contents were found to be 0 mole, 0 mole and0.33 mole. The amount of carboxyl groups in these organic acids totaled0.66 mole. The amount of the compound having an alcoholic hydroxylgroup, added at the initial stage and the subsequent stages, was 102grams for PEG#300, and the amount of the alcoholic hydroxyl groupstotaled 0.68 mole.

Example 3

[0117] The experimentation of repetition of depolymerization reactionswas initiated as in example 1. After the 10th depolymerization reactionwas finished, the glycolic acid oligomer to be added was changed toglycolic acid oligomer (c) obtained in synthesis example 3. At the 11thand subsequent cycles, the depolymerization reactions were successivelycarried out with the addition of 60 grams of glycolic acid oligomer (c)and 150 grams of TEGDME. Until the 25th depolymerization reaction, therewas no drop of the distillation rate of glycolide; the depolymerizationreactions could be carried out stably at 11 to 12 grams/hour.

[0118] Upon the completion of the 25th reaction, the feed of glycolicacid oligomer added up to 1,540 grams. The depolymerization reactionsystem was hydrolyzed under alkaline conditions to analyze the contentsof diglycolic acid, methoxy acetic acid and oxalic acid present therein.As a result, the respective contents were found to be 0.26 mole, 0.13mole and 0 mole. The amount of carboxyl groups in these organic acidstotaled 0.65 mole. The amount of PEG#300 added at the initial stage was42 grams, and the amount of lauryl triethylene glycol contained asco-condensed in glycolic acid oligomer (c) was 128 grams. The amount ofthe respective alcoholic hydroxyl groups totaled 0.68 mole.

Example 4

[0119] A 500-ml flask was charged with 100 grams of glycolic acidoligomer (c) obtained in synthesis example 3 and 200 grams of TEGDMEacting as the polar organic solvent. The flask was heated up to 260° C.while the inside pressure was lowered to 8.0 kPa. Glycolic acid oligomer(c) was put in a dissolved state. A distilled-out solution was cooledwith ice water for entrapment.

[0120] After a five-hour reaction, the amount of the distilled-outsolution was 268 grams. From this solution, 0.59 grams of glycolide and150 grams of TEGDME were recovered. With 60 grams of glycolic acidoligomer (c) and 150 grams of TEGDME added to the reaction solutionremaining in the flask, the depolymerization reaction was carried outunder the same conditions as mentioned above. In this way, thedepolymerization reaction was repetitively carried out with the additionof glycolic acid oligomer (c) in the weight corresponding to the amountof glycolide obtained in the distilled-out solution and TEGDME in thesame amount as that of TEGDME in the distilled-out solution. Until the25th depolymerization reaction, there was no drop of the distillationrate of glycolide; the distillation rate of glycolide was kept stable at11 to 12 grams/hour, indicating that the stable yet efficientdepolymerization reactions could be carried out over an extended period.

[0121] Upon the completion of the 25th reaction, the feed of glycolicacid oligomer added up to 1,540 grams. The depolymerization reactionsystem was hydrolyzed under alkaline conditions to analyze the contentsof diglycolic acid, methoxy acetic acid and oxalic acid present therein.As a result, the respective contents were found to be 0.26 mole, 0.13mole and 0 mole. The amount of carboxyl groups in these organic acidstotaled 0.65 mole. The amount of lauryl triethylene glycol contained asco-condensed in glycolic acid oligomer (c) was 220 grams, and the amountof alcoholic hydroxyl groups was 0.69 mole.

Comparative Example 1

[0122] A 500-ml flask was charged with 100 grams of glycolic acidoligomer (a) obtained in synthesis example 1, 200 grams of TEGDME actingas the polar organic solvent and 42 grams (corresponding to 0.28 mole ofalcoholic hydroxyl group) of PEG#300 having an average molecular weightof 300 and a boiling point of 410 to 470° C. The flask was heated up to260° C. while the inside pressure was lowered to 8.0 kPa. Glycolic acidoligomer (a) was put in a dissolved state, and the distllled-outsolution was cooled with ice water for entrapment.

[0123] After a five-hour reaction, the amount of the distilled-outsolution was 210 grams. From this solution, 58 grams of glycolide and150 grams of TEGDME were recovered. The distillation rate of glycolidewas 11 to 12 grams/hour. With 60 grams of glycolic acid oligomer (a) and150 grams of TEGDME added to the reaction solution remaining in theflask, the depolymerization reaction was carried out under the sameconditions as mentioned above.

[0124] In this way, the depolymerization reaction was repetitivelycarried out with the addition of glycolic acid oligomer (a) in theweight corresponding to the amount of glycolide obtained in thedistilled-out solution and TEGDME in the same amount as that of TEGDMEin the distilled-out solution. The depolymerization reaction wasrepeated ten times. The feed of glycolic acid oligomer (a) added up to640 grams. At this time, the distillation rate of glycolide decreaseddown to 8 to 9 grams/hour. Then, the 11th depolymerization reaction wascarried out with the addition of 60 grams of glycolic acid oligomer (a)and 150 grams of TEGDME. Consequently, the distillation rate ofglycolide decreased further. Likewise, the 12th to 15th depolymerizationreactions were successively carried out with the addition of 60 grams ofglycolic acid oligomer (a) and 150 grams of TEGDME. However, as thenumber of repetition increased, the distillation rate of glycolidebecame even much lower. At the 15th depolymerization reaction, thedistillation rate of glycolide jumped down to 3 to 4 grams/hour.

[0125] Upon the completion of the 15th reaction, the feed of glycolicacid oligomer (a) added up to 940 grams. The depolymerization reactionsystem was hydrolyzed under alkaline conditions to analyze the contentsof diglycolic acid, methoxy acetic acid and oxalic acid present therein.As a result, the respective contents were found to be 0.16 mole, 0.08mole and 0 mole. The amount of carboxyl groups in these organic acidstotaled 0.40 mole. The amount of the compound having an alcoholichydroxyl group, added at the initial stage, was 42 grams for PEG#300,and the amount of the alcoholic hydroxyl groups totaled 0.28 mole.

INDUSTRIAL APPLICABILITY

[0126] The present invention provides a process for producing glycolideby depolymerization by heating of glycolic acid oligomers wherein, byimparting long-term stability to a depolymerization reaction systemcontaining a glycolic acid oligomer, depolymerization reactions can berun stably yet with efficiency, even when the de-polymerizationreactions are carried out over an extended period.

[0127] The present invention also provides a unheard-of glycolic acidoligomer capable of reducing or substantially eliminating adverseinfluences ascribable to impurities contained in glycolic acid that isthe raw material for glycolic acid oligomers, so that depolymerizationreactions can be carried out stably yet with efficiency, even when theyare run continuously or repeatedly over an extended period.

1. A glycolide production process including a step of depolymerizationby heating of a glycolic acid oligomer, wherein: a depolymerizationreaction is carried out through the following steps (i) to (iv): step(i) of heating a depolymerization reaction system comprising a glycolicacid oligomer or a glycolic acid oligomer plus a polar organic solventto depolymerize the glycolic acid oligomer into glycolide, step (ii) ofdistilling the glycolide formed by depolymerization or the glycolide andpolar organic solvent out of the depolymerization reaction system, step(iii) of recovering the glycolide from distillates obtained bydistillation, and step (iv) of charging the glycolic acid oligomer orthe glycolic acid oligomer and polar organic solvent continuously orintermittently-into the depolymerization reaction system, in which: (v)during the depolymerization reaction, a compound (A) having an alcoholichydroxyl group is permitted to exist in the depolymerization reactionsystem, provided that the amount of the compound (A) in thedepolymerization reaction system is controlled such that the alcoholichydroxyl group amount of said compound (A) is kept at 0.5 equivalent orgreater with respect to the total carboxyl group amount of an organicacid (B) comprising diglycolic acid, methoxy acetic acid and oxalic acidformed upon hydrolysis of the depolymerization reaction system underalkaline conditions.
 2. The production process according to claim 1,wherein the depolymerization reaction is carried out continuously orrepeatedly in said depolymerization reaction system.
 3. The productionprocess according to claim 1, wherein during the depolymerizationreaction, the amount in the depolymerization reaction system of thecompound (A) having an alcoholic hydroxyl group is controlled such thatthe alcoholic hydroxyl group amount of said compound (A) is kept at 0.9equivalent or greater with respect to the total carboxyl group amount ofsaid organic acid (B).
 4. The production process according to claim 1,wherein the compound (A) having an alcoholic hydroxyl group has aboiling point of 190° C. or higher.
 5. The production process accordingto claim 1, wherein the compound (A) having an alcoholic hydroxyl groupis added into the depolymerization reaction system, whereby, during thedepolymerization reaction, the amount of said compound (A) in thedepolymerization reaction system is controlled such that the alcoholichydroxyl group amount of said compound (A) is kept at 0.5 equivalent orgreater with respect to the total carboxyl group amount of said organicacid (B).
 6. The production process according to claim 1, wherein aglycolic acid oligomer obtained by condensation of glycolic acid in thepresence of the compound (A) having an alcoholic hydroxyl group is usedas the glycolic acid oligomer, whereby, during the depolymerizationreaction, the amount of said compound (A) in the depolymerizationreaction system is controlled such that the alcoholic hydroxyl groupamount of said compound (A) is kept at 0.5 equivalent or greater withrespect to the total carboxyl group amount of said organic acid (B). 7.The production process according to claim 6, wherein the compound (A)having an alcoholic hydroxyl group has a boiling point of 190° C. orhigher.
 8. The production process according to claim 1, wherein thecompound (A) having an alcoholic hydroxyl group is at least one compoundselected from the group consisting of an alkylene diol, a polyalkyleneglycol and a polyalkylene glycol monoether.
 9. The production processaccording to claim 1, wherein at said step (i) the depolymerizationreaction system comprising a glycolic acid oligomer and a polar organicsolvent is heated to depolymerize the glycolic acid oligomer intoglycolide, at said step (ii) the glycolide formed by depolymerizationand the polar organic solvent are distilled out of the depolymerizationreaction system, and at said step (iii) the glycolide is separated andrecovered from distillates obtained by distillation.
 10. The productionprocess according to claim 9, wherein at said step (i) adepolymerization reaction system comprising a mixture containing aglycolic acid oligomer and a polar organic solvent having a boilingpoint in the range of 230 to 450° C. is heated under normal pressure orreduced pressure in such a state that the proportion of a remainingglycolic acid oligomer melt phase in said mixture is 0.5 or less, andheating is continued in that state to depolymerize the glycolic acidoligomer into glycolide.
 11. The production process according to claim10, wherein the polar organic solvent has a molecular weight in therange of 150 to
 450. 12. The production process according to claim 10,wherein the polar organic solvent is a polyalkylene glycol diether. 13.The production process according to claim 10, wherein said mixturefurther contains the compound (A) having an alcoholic hydroxyl group.14. The production process according to claim 9, wherein at said step(iv), the glycolic acid oligomer and polar organic solvent in freshamounts that make up for the amounts of distillates distilled out of thedepolymerization reaction system are continuously or intermittentlycharged into the depolymerization reaction system.
 15. The productionprocess according to claim 9, wherein the compound (A) having analcoholic hydroxyl group is continuously or intermittently charged intothe depolymerization reaction system, whereby, during thedepolymerization reaction, the amount of said compound (A) in thedepolymerization reaction system is controlled such that the alcoholichydroxyl group amount of said compound (A) is kept at 0.5 equivalent orgreater with respect to the total carboxyl group amount of said organicacid (B).
 16. The production process according to claim 9, wherein aglycolic acid oligomer obtained by condensation of glycolic acid in thepresence of the compound (A) having an alcoholic hydroxyl group iscontinuously or intermittently charged into the depolymerizationreaction system, whereby, during the depolymerization reaction, theamount of said compound (A) in the depolymerization reaction system iscontrolled such that the alcoholic hydroxyl group amount of saidcompound (A) is kept at 0.5 equivalent or greater with respect to thetotal carboxyl group amount of said organic acid (B).
 17. A glycolicacid oligomer for the production of glycolide, which is obtained bycondensation of glycolic acid in the presence of a compound (A) havingan alcoholic hydroxyl group and a boiling point of 190° C. or higher aswell.
 18. The glycolic acid oligomer for the production glycolideaccording to claim 17, which is a glycolic acid oligomer obtained bycondensation of said glycolic acid in the presence of the compound (A)in such an amount that the alcoholic hydroxyl group amount of saidcompound (A) is in the range of 0.5 to 1.5 equivalents with respect tothe total carboxyl group amount of an organic acid (B) contained in thestarting glycolic acid and comprising diglycolic acid, methoxy aceticacid and oxalic acid.
 19. The glycolic acid oligomer for the productionof glycolid according to claim 17, wherein the compound (A) having analcoholic hydroxyl group is at least one compound selected from thegroup consisting of an alkylene diol, a polyalkylene glycol and apolyalkylene glycol monoether.